I suspect that is not paint there; but likely to be one of the unlimited designs available with removable (aka peel and stick) wallpaper or wall mural. Most of the online stores that sell those, also allow customers to upload any design and they custom print the paper according to your exact size needed. I love them because it's super easy (much easier than trying to paint that) and comparatively inexpensive (money and time) way to get exactly what you want. Plus, removable papers and murals are self-sticking (no pasting required) and can be removed easily and placed elsewhere over and over again. Even when you move you can just peel and take with you to re-adhere in your new home. It's very durable stuff. I recommend you check out rebelwalls.com; limitlesswalls.com; eazywallz.com or any of the many others.

I would do the following:

1. Paint the entire wall white.

   

2. Pick the two darkest colors of the stripes and paint them with a gap in between

   

3. Repaint half of the first gap and while the paint is still wet, take a DRY brush and feather the white paint about 1/3 the way into each of the other two colors. When doing this, start the middle of the gap and drag towards the middle of the next color. Feather the wet paint until you have achieved the desired look.

   You might have to use a spray bottle of water so that the painter feathers more easily.

   

4. Repeat until the entire wall is complete.

I suspect an amateur would have a difficult time getting the look pictured, without a lot of practice! A poorly done paint job would be a waste of time and materials.

I agree that it is most likely wallpaper or a mural.